The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. Particularly, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera to be built in a portable device including a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called smartphones have been more widely used, i.e., cellular phones with such functions as those of portable information terminals (PDA) and/or personal computers. Since the smartphones generally are highly functional as opposed to the cellular phones, it is possible to use images taken by a camera thereof in various applications.
Generally speaking, product groups of cellular phones and smartphones often include various designs from the ones for beginners to the ones for advanced users. Among them, an imaging lens to be mounted in a product, which is developed for advanced users, is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element of these days.
In order to achieve a high resolution imaging lens, there is a method of increasing the number of lenses that compose the imaging lens. However, increase of the number of lenses easily causes increase in the size of the imaging lens, so that it is not preferred for the imaging lens to be mounted in a small-sized camera, such as the ones of cellular phones and smartphones. For this reason, the imaging lens has been conventionally developed in view of restraining the number of lenses as small as possible.
In recent years, a technology for increasing a pixel count of an imaging element has been dramatically advanced. A main focus in developing the imaging lens has been changing to achieving a lens configuration having high resolution, rather than shortening a total track length. For example, there has been available a camera unit capable of obtaining a quality image even in comparison with that taken by a digital still camera by attaching the camera unit to a cellular phone, a smartphone, or the like, instead of mounting the camera unit that includes an imaging lens and an imaging element in a cellular phone, a smartphone, or the like, which is conventionally done.
Although a seven-lens configuration could be slightly disadvantageous in view of downsizing the imaging lens due to the large number of lenses that compose the imaging lens, there is high flexibility in designing. Therefore, the seven-lens configuration may have potential to achieve satisfactory correction of aberrations and downsizing of the imaging lens in a balanced manner. As the imaging lens having such a seven-lens configuration, for example, there is known one that is described in Patent Reference.
Patent Reference: Japanese Patent Application Publication No. 2012-155223
According to Patent Reference, the conventional imaging lens includes a first lens having a biconvex shape; a second lens that is joined to the first lends and has a biconcave shape; a third lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to the object side; a fourth lens that is positive and has a shape of a meniscus lens directing a concave surface thereof to the object side; a fifth lens that is negative and has a convex surface directing to the object side; a sixth lens having a biconvex shape; and a seventh lens having a biconcave shape, arranged in the order from the object side. According to the conventional imaging lens disclosed in Patent Reference, through restraining a ratio between a focal length of a first lens group composed of the lenses from the first lens to the fourth lens and a focal length of a second lens group composed of the lenses from the fifth lens to the seventh lens within a certain range, it is possible to attain downsizing of the imaging lens and satisfactory correction of aberrations.
According to the conventional imaging lens disclosed in Patent Reference, although the size of the imaging lens is small, correction of an image surface is not sufficient and especially distortion is relatively large. Therefore, there is a limit by itself in achieving a high-resolution imaging lens. According to the lens configuration of the conventional imaging lens described in Patent Reference, it is difficult to more satisfactorily correct aberrations while downsizing the imaging lens.
Here, the above-described problem is not only specific to the imaging lens to be mounted in cellular phones and smart phones. Rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain both downsizing and satisfactory correction of aberrations.
Further objects and advantages of the invention will be apparent from the following description of the invention.